1. Field of the Invention
The present invention relates to a circuit board used in electrical and electronic equipment and a method for manufacturing the same. In particular, the present invention relates to a method for manufacturing a circuit board suitable for a power electronics field that uses a relatively large current.
2. Description of the Related Art
In recent years, the improvement in density and function of a semiconductor or the like has been emphasized with a demand for high performance and miniaturization of electronic equipment. This leads to a need for a small high-density circuit board, on which the semiconductor or the like is mounted. Thus, an important design consideration is thermal dissipation of the circuit board. To improve the thermal dissipation, a circuit board that can increase the thermal conductivity of an insulating substrate and suppress a local temperature rise better than a conventional printed circuit board made of glass-epoxy resin is desired. A metal-based circuit board is known to have higher thermal conductivity than that of the glass-epoxy resin. The metal-based circuit board includes a metal plate of copper or aluminum and a circuit pattern formed on one surface of the metal plate via an insulating layer. However, it is difficult to reduce the size and weight of the metal-based circuit board because the metal plate has a relatively large thickness. Moreover, the insulating layer should be made thinner to enhance the thermal conductivity of the circuit board, causing the problems of decreasing a withstand voltage and increasing a stray capacitance. A ceramic substrate and a glass-ceramic substrate have higher thermal conductivity than that of the glass-epoxy substrate. However, they use metal powder or a material obtained by firing the metal powder as a conductive material. Therefore, the wiring resistance becomes relatively high, which increases loss and Joule heat generated in using a large current.
A circuit board with high thermal conductivity that can solve the above problems and be manufactured in a similar process to that of a conventional glass-epoxy substrate is disclosed, e.g., in JP 10-173097A. FIGS. 7A to 7D show a method for manufacturing the thermal conductive circuit board. A slurry that includes an inorganic filler and a thermosetting resin is formed into a sheet-shaped thermal conductive mixture 71. Then, the thermal conductive mixture 71 is dried and sandwiched between metal foils 72, as shown in FIG. 7A. Next, the thermal conductive mixture 71 is cured by applying heat and pressure to form an insulating layer 73, as shown in FIG. 7B. Then, holes 74 are processed as shown in FIG. 7C, and connections are made between the layers by copper plating. Finally, as shown in FIG. 7D, a circuit pattern 75 is formed, resulting in a thermal conductive circuit board.
When a circuit board is produced in this manner, drilling is employed generally to form the holes. However, when the thermal conductive mixture includes a high proportion of inorganic filler to enhance thermal conductivity, a drill is worn significantly due to the hard inorganic filler in the insulating material. Moreover, chipping or the like occurs easily, and thus the hole quality often is degraded. If the drill is replaced at frequent intervals to prevent wear, the productivity is reduced greatly and the cost for processing holes is increased as compared with the general printed circuit board.
When holes are formed in the thermal conductive mixture with its thermosetting resin uncured, and then the thermal conductive resin mixture and the metal foils are bonded while applying heat and pressure for curing, the hole shape cannot be maintained because the thermal conductive mixture is softened by heating. This makes it difficult to form the holes, so that the subsequent plating is impossible.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a method for manufacturing a circuit board with high thermal dissipation that can achieve improved productivity and low cost in processing holes.
A method for manufacturing a circuit board of the present invention includes the following: a first step of preparing a thermal conductive resin composition including 70 to 95 mass % of an inorganic filler and 5 to 30 mass % of a resin composition that includes a thermosetting resin in the uncured state, a thermoplastic resin, and a latent curing agent; a second step of preparing a circuit board precursor by heating the thermal conductive resin composition at a temperature lower than a temperature at which the thermosetting resin in the thermal conductive resin composition starts to cure while applying pressure so that the thermal conductive resin composition is solidified irreversibly; a third step of providing holes through the circuit board precursor; and a fourth step of curing the thermosetting resin in the circuit board precursor.
It is understood that mass % is equivalent to weight %.
In the method of the present invention, the second step may prepare the circuit board precursor by bringing the thermal conductive resin composition into contact with a metal foil and bonding the thermal conductive resin composition and the metal foil together by heating at a temperature lower than the temperature at which the thermosetting resin in the thermal conductive resin composition starts to cure while applying pressure so that the thermal conductive resin composition is solidified irreversibly. The method further may include a step of forming a circuit pattern by processing the metal foil.
In the method of the present invention, the second step may prepare the circuit board precursor by sandwiching the thermal conductive resin composition between two metal foils and bonding the thermal conductive resin composition and the metal foils together by heating at a temperature lower than the temperature at which the thermosetting resin in the thermal conductive resin composition starts to cure while applying pressure so that the thermal conductive resin composition is solidified irreversibly. The method further may include steps of providing through holes by plating the holes with copper to make an electrical connection between the metal foils and forming a circuit pattern by processing the metal foils.
According to these methods, the thermal conductive resin composition can be solidified with its thermosetting resin uncured and bonded to the metal foil. Therefore, the circuit board can be processed into a desired plate shape. Moreover, the holes can be formed in a simple manner because processing after the solidification is easy. Even if the thermosetting resin is cured after forming the holes, it is not necessary to apply pressure, unlike a conventional printed circuit board. Thus, the holes are not deformed or filled, and a change in the size caused by curing and shrinkage can be reduced.
In the method of the present invention, it is preferable that a step of making the thermal conductive resin composition integral with a reinforcing material is performed after the first step. This improves the strength of the circuit board. Moreover, when the thermal conductive resin composition is bonded to the metal foil, the thickness of the insulating layer can be maintained because of the reinforcing material, thereby preventing excessive effusion of the thermal conductive resin composition.
In the method of the present invention, it is preferable that the reinforcing material is made of at least one selected from a ceramic fiber and a glass fiber. These materials have high strength and relatively excellent thermal conductivity.
In the method of the present invention, it is preferable that the application of heat and pressure in the second step is performed under vacuum. This can reduce voids in the circuit board and prevent degradation of the metal foil.
In the method of the present invention, it is preferable that the curing in the fourth step is performed by applying heat and pressure. This can improve adhesion between the metal foil and the insulating substrate.
In the method of the present invention, it is preferable that the thermal conductive resin composition has a viscosity of 100 to 100000 Paxc2x7s, and the thermal conductive resin composition in its irreversible solid state has a viscosity of 8xc3x97104 to 3xc3x97106 Paxc2x7s. These viscosity ranges make it easier to process the thermal conductive resin composition into a substrate and to form the holes in the thermal conductive resin composition when it is solidified.
In the method of the present invention, it is preferable that the holes in the third step are processed with one selected from the group consisting of a punching machine, a punching die and a drill.
In the method of the present invention, it is preferable that at least a portion of the exterior of the circuit board precursor is processed to be a desired shape while processing the holes in the third step.
In the method of the present invention, it is preferable that the metal foil is a copper foil having a thickness of 12 to 200 xcexcm, and at least one surface of the copper foil is made uneven.
In the method of the present invention, it is preferable that the temperature at which the thermal conductive resin composition is solidified irreversibly is 70 to 140xc2x0 C.
In the method of the present invention, it is preferable that the inorganic filler includes at least one selected from the group consisting of Al2O3, SiO2, MgO, BeO, Si3N4, SiC, AlN and BN.
A circuit board of the present invention includes an insulating substrate obtained by curing a thermal conductive resin composition, the thermal conductive resin composition including (a) 70 to 95 mass % of an inorganic filler and (b) 5 to 30 mass % of a resin composition that includes a thermosetting resin in the uncured state, a thermoplastic resin, and a latent curing agent. A circuit pattern is formed on at least one surface of the insulating substrate, and holes are formed to penetrate both surfaces of the insulating substrate.
In the circuit board of the present invention, it is preferable that the insulating substrate is obtained by curing the thermal conductive resin composition that is integral with a reinforcing material including at least one selected from a ceramic fiber and a glass fiber.
A power conversion module of the present invention includes a circuit board manufactured by the method according to the present invention, a semiconductor, and a passive component. The semiconductor and the passive component are mounted on the circuit board. Here, the power conversion module is referred to as a module that converts voltage and electric power by itself or by being connected to an auxiliary circuit. Examples of the power conversion module include a DC-CD converter and an inverter.